2
One logically asks the underlying questions: What is the design objective? What affects the achievement of the objective?  Peak flows? Average Velocity? Volume? Are the design criteria specified by regulation? Are the specified design criteria appropriate for the design objective? How should the design flow be selected? How were the original sewers designed?

4
What were the original sewer objectives? Get the muck out of the street  road gutters, deep enough to convey rain  if streets flooded too often, deepen the gutters or build elevated walkways Get the urban effluvium out of the streets

5
Metcalf and Eddy, 1914 “Public latrines were doubtless used by most of the people and it is probable that the gutters were the chief receptacle of the ordure of the city, which washed thence into the sewers.”

6
Cholera outbreaks in the middle of the 19 th century changed that Early proponents of dual sewers suggested  Initially build smaller, less expensive sewers or household waste  Later build larger street drainage But what happened to the street muck?

7
Early sewer criteria were conflicting Sewers were to be large enough (> 2 meters) for easy cleaning Sewers were to be small enough for economical construction

8
And early sewer sizing was not based on science “American sewerage practice is noteworthy among the branches of engineering for the preponderating influence of experience rather than experiment upon the development of many of its features” Metcalf and Eddy, 1914

9
20 th Century engineering texts and training called for First, calculate the expected flow  size and nature of the area collected  times a design flow per unit area of a given nature (gpd/residential acre)  times a peaking factor Second, size to convey with adequate velocity (Manning's)  V=ƒ(P,Q p,S) where V= velocity P = wetted perimeter Q p = flow S = slope

20
Sewer network design flows, however, are not simply related to rain Peak sewer flows  Does the peak rain coincide with peak dry weather flow?  What is the effective tributary area? Is it changing?  What is the effective time of concentration?  Does the design event come in the dry or wet season?

22
Sewer flow design events also consider What values are at risk 2 ? How do sewer design flows affect those risks and values? What target level of risk will protect the values? What design event will achieve best protect the values? 2 WEF Guide to Managing Peak Wet Weather Flows (Nov. 2006)

23
MSD of Greater Cincinnati experience with ‘risk to values’ design storm selection Community values and measures were imputed from stakeholder meetings Four test storms were defined  all had similar distribution, duration, antecedent conditions and shape  frequency was associated with storm frequency for a specific duration Sewer relief projects were defined for each test storm Values achieve with the “relieved” sewers were defined 3 Johnson, R. et. al., Design-Storm Analysis Extrapolated to Estimate Long- Term Performance. WEFTEC06.

25
One logically asks the underlying questions: What is the design objective? What affects the achievement of the objective?  Peak flows? Average Velocity? Volume? Are the design criteria specified by regulation? Are the specified design criteria appropriate for the design objective? How should the design flow be selected? How were the original sewers designed?